La0.9Sr0.1Ga0.8Mg0.2O3-x (LSGM) was additionally doped with 0.1 to 0.3 mol Co. Powders and pellets were prepared by a solid-state reaction in air. X-ray diffraction shows a cubic structure for y less than or equal to 0.1 mol Co and an hexagonal structure for y greater than or equal to 0.2 mol Co. At p(O2) < 10(-6) Pa the structure of all compositions changed to cubic, independent of Co content. Oxygen stoichiometries of the reduced and oxidized states of the mixed oxides were determined by solid electrolyte coulometry. The Co2+/Co4+ concentrations were calculated from oxygen stoichiometries. At y = 0.1 mol Co is stabilized at the Ga site as Co2+ up to high p(O2). The electrical conductivity of La0.9Sr0.1(Ga0.9Co0.1)(0.8)Mg0.2O3-x divided by delta shows a wide ionic domain. With increasing y the conductivity increases due to additional p-type conduction. A defect model including electron holes and oxygen vacancies is discussed.